1. Field of the Invention
The present invention relates to an axial fan having a hub portion having a rotating center and blades arranged on the outer periphery of the hub portion, and a method of designing the blades of the axial fan.
2. Description of the Related Art
An axial fan (for example, propeller fan) for sucking gas in an axial direction and then blowing out the air in the axial direction is applied to an outdoor unit of an air conditioner, a ventilation fan, an electric fan or the like. The axial fan is equipped with a hub portion having the rotating center and a plurality of blades arranged on the outer periphery of the hub portion. Each blade is designed in a three-dimensional curved-surface shape (for example, see Japanese Patent No. 3,754,244).
In order to enhance the structural rigidity of this type of axial fan, the thickness of each blade may be increased. However, if the thickness of each blade is increased, the whole weight of the fan itself is increased, and centrifugal force acting on the fan itself is increased, so that the strength to the centrifugal force is reduced. On the other hand, when the rotational number of a fan motor is suppressed under control in order to reduce the centrifugal force acting on the fan, there occurs a problem that the air blowing performance of the fan is greatly reduced.
Furthermore, this type of axial fan has a noise problem that noise occurs at the outer peripheral side of the axial fan due to blade tip vortex occurring at the outer peripheral side of each blade or the like. In order to suppress this blade tip vortex, it has been proposed to partially change the shape of the blade to provide an additional blade to a basic blade (For example, JP-2005-105865).
When the blade of this type of axial fan is designed, the cross-sectional shape in the peripheral direction of the blade and the cross-sectional shape in the radial direction of the blade are defined by using mathematical formulas defined by several parameters, and the blade is designed by using these mathematical formulas (for example, Japanese Patent No. 3,754,244). This design method is applied to a blade having a three-dimensional curved surface which has no additional blade, and it has been difficult to partially change the shape of the blade. Therefore, a work of designing a blade having an additional blade is complicated, and also it is difficult to assess the best blade shape.